Ideas concerning the possibilities of implanted sensory prostheses have existed for almost two centuries. During the recent portion of this period, auditory prostheses were developed and later improvements were suggested by a number of investigators. Most auditory work has focused on the problem of developing a cochlear implant to directly stimulate auditory nerve fibers existing from the cochlea, although during a few investigations, electrodes have been implanted on auditory cortex. Visual prostheses were also heretofore developed with later improvements being suggested by a number of investigators. In these investigations, electrodes have been implanted on the surface of the visual cortex, thereby permitting direct stimulation of the central nervous system. In addition, a number of investigators have used animal model systems to study the anatomical and physiological consequences of using chronic implants.
It has been generally accepted that the natural sensory system is quite complex and development of sensory prostheses did not therefore prove to be an easy task. Considering the fact that the auditory nerve, for example, has approximately 30,000 nerve fibers while most auditory prostheses have had only one channel and only a few have had even a dozen channels, and further considering that the optic nerve has approximately one million nerve fibers while most visual prostheses have had only a few dozen channels, the likelihood of such prostheses having any significant benefit to patients seemed initially very small. But the performance of even the earliest prosthetic devices has demonstrated that patients could derive considerable benefit from artificial prostheses. If one is totally deprived of input from a given sensory modality, any additional information obtained via that modality will represent a tremendous improvement even if the prosthetic performance level is substantially below the performance level permitted by an intact natural sensory system.
Despite this progress in prosthetic research, two important problems still existed. One problem stems from the disparate performance of visual and auditory prostheses in the area of pattern recognition, while the second problem arises from the high current levels now necessary for pattern recognition in visual prostheses.
With respect to the first problem, visual prostheses have led to very successful pattern recognition. Patients fitted with these devices have been able to "read" Braille patterns produced by direct stimulation of their visual cortex at rates that compare well with the rates with which they "read" Braille with their finger tips. Auditory prostheses have, however, not yet led to any useful ability to recognize the auditory patterns used for speech communication. With both single channel and multichannel cochlear implants, patients have not been able to recognize randomly spoken words to any reliable degree. This is not to say, however, that such patients do not extract any information from speech. When given a limited set of words from which to choose, patients do significantly better than chance at associating a given spoken word with the corresponding member of the set. This is encouraging and suggests that these devices can be improved to mediate speech perception. At least some patients wearing these devices have expressed a feeling that the information they obtained from the cochlear prosthesis is almost good enough to understand speech. Such patients have been able to recognize the difference between speech and non-speech sounds, and even distinguish one speaker from another, but they cannot understand a spoken message.
With respect to the second problem, even though visual cortex implants have led to successful pattern recognition, the current level necessary for the production of a phosphene (i.e., a perceptible spot of light evoked by the prosthesis) is fairly high. Research on animal models indicates that sustained application of currents large enough to produce phosphene detection may also tend to produce temporary and permanent changes in brain structure and function. There is, therefore, some question at the present time about the feasibility of using visual prostheses for extended periods of time without injuring the brain.